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UPWELLING AND DOWNWELLING 2006 IRIS 5-YEAR PROPOSAL Structural Features and Velocity Structures of the “African Anomaly” Yi Wang, Lianxing Wen • SUNY at Stony Brook The IRIS PASSCAL experiments have supplied the seismology community with highquality, freely available and spatially dense datasets. The high-quality broadband seismic data recorded in three PASSICAL experiments in Africa (the Tanzania, the Kaapvaal, and the Kenya/Ethiopia experiments) revealed a very-low velocity province occupying from the South Atlantic Ocean to the Indian Ocean in the lowermost mantle and locally extending 1300 km above the core-mantle boundary beneath southern Africa (we term it the “African anomaly”). The base of the “African anomaly” exhibits an L-shaped form changing from a north-south orientation in the South Atlantic Ocean to an east-west orientation in the Indian Ocean and occupies an area of about 1.8x107 km2 and a volume of about 4.9x109 km3. The dense seismic data and the development of hybrid method also revealed detailed structural features of the “African anomaly”. The base of the “African anomaly” has rapidly varying thicknesses from 300 km to 0 km, steeply dipping edges and a linear gradient of shear velocity reduction from -2% (top) to -9% to -12% (bottom) relative to the Preliminary Reference Earth Model (Wen et al., 2001; Wen, 2001; Wen, 2002; Wang and Wen, 2004), Its extension into the mid-lower mantle beneath southern Africa exhibits a “cusp-like” geometry with both flanks dipping toward its apex and its lateral dimension increasing with depth. The average shear velocity decreases are –5% in the base and -2% to -3% in the portion in the mid-lower mantle, and an S- to P- velocity perturbation ratio (δ InV S /δ InV P ) is 3:1 for the (a) Geometry (green contour) and ray paths of the seismic phases used to constrain the geometry and S-velocity structure of the “African anomaly” in a 2-D cross section along the East Pacific Rise (EPR), Drake Passage (DP), South Sandwich Islands (SS), Iran (IR), and Hindu Kush (HK). Black and red traces represent propagation paths without and with observed travel time delays that can be attributed to the “African anomaly”, respectively. Black stars represent seismic events. Seismic arrays and earthquake locations are denoted at the top of the Earth’s surface. (b) Map view of great-circle paths (gray traces), locations of earthquakes (red stars) and seismic arrays (black triangles). The thick green dashed curve represents the 2-D cross section shown in (a) and the thick black contour is the geographic boundary of the base of the “African anomaly”shown in (c). (c) Threedimensional views of structure features and velocity structure of the base of the “African anomaly” (vertical exaggeration: 5.55, viewed from N). entire “African anomaly” (Wang and Wen, 2005). Our ability to constrain both the geometry and velocity structures of the “African anomaly” brought significant progress in understanding the origin and dynamics of the anomaly (Wen et al., 2001). The structural features and velocity structures of the “African anomaly” unambiguously indicate that the “African anomaly” is compositionally distinct and geologically stable. It has also been suggested that the velocity structure in the lowermost mantle can best be explained by partial melt driven by a compositional change produced in the early Earthʼs history (Wen et al., 2001). Wen, L., Seismic evidence for a rapidly varying compositional anomaly at the base of the Earthʼs mantle beneath the Indian Ocean, Earth Planet. Sci, Lett., 194, 83 – 95, 2001. Wen, L., P. Silver, D. James, and R. Kuehnel, Seismic evidence for a thermo-chemical boundary layer at the base of the Earthʼs mantle, Earth Planet. Sci. Lett., 189, 141 – 153, 2001. Wen, L., An SH hybrid method and shear velocity structures in the lowermost mantle beneath the central Pacific and South Atlantic oceans, J. Geophys. Res., 103, doi:10.1029/ 2001JB004999, 2002. Wang, Y., and L. Wen, Mapping the geometry and geographic distribution of a very low velocity province at the base of the Earthʼs mantle, J. Geophys. Res., 109, doi:10.1029/ 2003JB002674, 2004. Wang, Y., and L. Wen, Geometry and P- and S- velocity structures of the “African anomaly”, submitted to J. Geophys. Res., 2005. 138
2006 IRIS 5-YEAR PROPOSAL UPWELLING AND DOWNWELLING Seismic Evidence for Hotspot-Induced Buoyant Flow Beneath the Reykjanes Ridge James B. Gaherty • Lamont-Doherty Earth Observatory of Columbia University Volcanic hotspots and mid-ocean ridge spreading centers are the surface expressions of upwelling in Earthʼs mantle convection system, and their interaction provides unique information on upwelling dynamics. I investigated the influence of the Iceland hotspot on the adjacent mid-Atlantic spreading center (the Reykjanes Ridge, RR) using seismic surface waves from mid-Atlantic ridge earthquakes recorded at the Global Seismic Network station BORG and stations of the ICEMELT and HOTSPOT PASSCAL deployments (left). The surface waves from these events travel along and adjacent to the RR, and the travel times of these waves are sensitive to the average crust and upper-mantle velocity along each path. These delay times were inverted for age-dependent models of radial anisotropy (right). The models show a distinct pattern of shear anisotropy (∆V S ), with negative values (V SV > V SH ) above about 100 km depth, and positive values between about 100-200 km depth. This pattern of anisotropy is unlike that in comparable oceanic models, which display ∆V S > 0 throughout the upper 200 km of the mantle. This anisotropy suggests that the hotspot induces buoyancy-driven upwelling in the mantle beneath the ridge. In this model, the melt-zone upwelling is driven by buoyancy associated with retained melt, melt residuum, and/or locally hot (left) Bathymetric map of the North Atlantic study region. Surface waves of earthquakes (open circles) from the Reykjanes Ridge (RR) and the Gibbs Fracture Zone (GFZ) were recorded on Iceland at BORG and the ICEMELT and HOT- SPOT stations (inverted and upright triangles, respectively). Seafloor age is contoured at 20 Ma intervals. (right) Upper-mantle shear-velocity models of the Reykjanes region. Left panel displays mean shear speed (vS = (vSH+vSV)/2), while right panel displays shear anisotropy (vS = (vSH-vSV)/vS) in percent). Three age regions are shown: 0-5 Ma (short dash), 10-15 Ma (long dash), and 25-40 Ma (solid). Also shown (dash-dot lines) are shear-velocity models for Pacific upper-mantle (Nishimura and Forsyth, 1989). mantle. Such models produce a tight circulation within the melting zone, and as the mantle material moves out of the spreading center, a near-vertical fabric associated with the downgoing limb of the circulation is retained in the off-axis lithosphere to a depth of ~60-100 km. This result suggests that buoyancy-driven upwelling is an important component of ridge dynamics, especially in environments where passive sea-floor spreading is too slow to accommodate melt production. It also implies that the anisotropic structure of oceanic lithosphere may not be as simple as inferred through studies from the fast-spreading Pacific ridges, and that this structure holds important clues to ridge and plume dynamics. Gaherty, J.B., Seismic evidence for hotspot-induced buoyant flow beneath the Reykjanes Ridge, Science, 293, 1645-1647, 2001. 139
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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